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Watch Crop Stage For Optimum Weed Control

Getting a jump on the competition is a basic business principle; any advantage is better than none at all. In farming, your biggest competition is often weeds soaking up moisture and nutrients before or when the crop goes in. This competition directly steals yield out of the bin and takes away from your bottom line. Being aware of weed and crop stages helps you plan when and how to take out the competition.


There is a period of time every year in which weed competition must be controlled completely or yield loss will occur. Once this period passes you’ve got a bit of wiggle room, and weeds can be controlled when conditions are right without being concerned that the time passing is hurting the crop. This weed-free period can be divided into two sub periods. The first sub period is defined as the maximum length of time that can occur for the weeds that emerge with the crop to be allowed to grow before the weeds cause yield loss. This period is known as the critical weed-free period. The second sub period is the minimum time allowed for weeds that emerge after the crop must be removed before they cause yield loss. This is known as the critical weed removal period. If weeds can be controlled within these periods then a yield loss of more than 5 per cent can be prevented. Using this knowledge, you can spray using a non-residual post emergent chemical and take away the need, in some cases, for a season long residual chemical.

One of the easiest ways to monitor the critical weed-free period is to look at the crop staging. For spring cereals the critical weed free period occurs between the one to three leaf stages. For winter wheat a period of between 500 to 1,000 growing degree days is needed with a base temperature set at zero degrees. For forage crops the critical weed-free period is four to six weeks after planting in the year of establishment. Canola and other oilseeds need to be weed-free from emergence to the six leaf stage. Edible pulses have a varying critical period from the second and third node through to early maturity. (This is why residual management is so popular for this type of crop throughout the season). If weeds can be kept out of the crop for this period of time it will be very difficult to experience losses of over 5 per cent due to weed competition. Even late emerging weeds will have difficulty overtaking the crop due to their competitive advantage.


With this knowledge, it’s time to find balance between what works best for keeping the crop clean for the maximum period of time without going in too early for an in-crop weed control pass. The most important part of this may be the burn off before seeding. If the spring burn off is accomplished correctly and to the best possible result this give you the most time without weeds. This also allows you to time the in-crop application better for the minimum time of weeds in crop.

The difficult point with this, however, is that different crops react to different chemicals differently at certain stages of their life cycle. By giving yourself the most time after emergence without weeds, you maximize the probability of being able to spray in-crop at the best time to control weeds and minimize the chances of crop damage due to the spraying caused by drift, overlap and other types of miscalculations.


The critical period for weed control in canola is about the four leaf stage. For pulse crops the weed removal period starts as early as two weeks after emergence. For cereals, the herbicide used is a deciding factor for optimal spray timing. Between the flat leaf and flag leaf can work best depending on size and density of weeds, however once cereals get passed the flag leaf stage it’s really too late to spray — you’ve likely lost the yield already.

The critical weed removal period for pulses is from approximately the third node to the flowering stage of the plant. This larger window of time for critical weed control means that choosing to spraying based on chemical used for optimal weed control rather than crop stage is a good strategy. Crop safety is still a concern, however; don’t be counter-productive in the process and go in too early. Proper timing varies chemical by chemical — refer to your Guide to Crop Protection or talk to your chemical rep for correct timing windows.

Jay Peterson farms at Frontier, Sask.

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Jay Peterson farms near Frontier, Sask.

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